How to add API keys to Cursor without pasting them into chat

Install offprompt in the project with npx offprompt init, and Cursor's agent asks for a key with a tool call instead of in the chat. A page opens in your browser on 127.0.0.1, you paste the key there, and offprompt writes it to the project's .env. The agent gets back the key's name and a four-emoji fingerprint, never the value, so the key stays out of the conversation and out of every request sent to the model.

Adding a key with offprompt

Install offprompt in the project

In the project's folder, with Node 22 or later:

npx offprompt init --agent cursor

This copies the server to tools/offprompt/mcp.mjs and declares it in .cursor/mcp.json. Without --agent cursor, init sets up every agent it finds on your machine, Cursor included. Commit both, and everyone who opens the project in Cursor gets offprompt too. Check that offprompt is listed and turned on where Cursor's settings list MCP servers.

Keep the agent's file tools out of `.env`

Cursor's docs list .env* among the files Cursor ignores by default. To make it explicit, add the files to .cursorignore at the project's root, which keeps them from the Agent and Tab:

.cursorignore

.env
.env.*

Cursor's docs on ignore files say the Agent's terminal and MCP tools are not covered, so a command that prints the file still could.

Ask for something that needs a key

In the Agent, ask for the task, not the key:

Add Stripe Checkout to the pricing page.

The agent calls collect_secret for STRIPE_SECRET_KEY, says why it needs it, and names the file, .env unless the project reads another, such as .env.local.

Paste the key on the page

The page says who is asking, why, and which file it will write. The field links to where Stripe makes the key, and checks what you paste: a Stripe secret key starts with sk_live_, sk_test_, rk_live_ or rk_test_, so a key from the wrong provider is caught before it is written. Press the write button.

Compare the fingerprint

The page shows Written. and four emoji. The agent tells you the same four in the chat. When they match, the file holds exactly what you typed, and the agent carries on with the task, reading the key from the environment by name.

In a Cursor Cloud Agent

A Cloud Agent runs in a VM your browser is not on, so offprompt cannot open a page on your screen. The agent passes you a link instead, to the same page through a Cloudflare quick tunnel. Your browser encrypts the values to a key that only the sandbox holds before they leave it, so the tunnel carries ciphertext. Commit tools/offprompt/ and .cursor/mcp.json, and every Cloud Agent that clones the project has offprompt. See Cloud sandboxes.

Without offprompt

Ask the agent which variable it reads and from which file, then open the file in your editor and add the line yourself, with .cursorignore set as above. The value never passes through the chat, but you have to get the name and the file right, and a key the agent finds it needs halfway through a task stops the task until you do. How to give Claude Code an API key sets the ways to hand over a key side by side; they are the same for Cursor.

FAQ

Is it safe to paste an API key into Cursor's chat?

It is not. What you paste becomes part of the conversation: it goes to the model with every later request, stays in the chat's history, and can come back in a summary or in code the agent writes. A key that has been pasted cannot be taken back out. Rotate it at the provider.

Can Cursor's agent still read the `.env` file?

offprompt keeps the value out of the conversation on the way in. Afterwards the file is on disk. .cursorignore keeps the Agent's file tools and Tab out of it, and every offprompt result tells the agent not to open it and lists the keys the file holds, so the agent has no reason to. A terminal command can still print it. See the threat model.

Which keys does offprompt check?

offprompt's registry knows 65 providers and 111 kinds of key, such as OpenAI, Anthropic, Stripe, Supabase and Resend keys, with where each is made and the prefix and length it should have. Any other key gets a plain field, and a value that should be random, such as AUTH_SECRET, is generated on the page.

Does offprompt send my keys anywhere?

On your machine the value goes from your browser to offprompt, running on 127.0.0.1, and into the file. From a Cloud Agent it crosses a Cloudflare tunnel encrypted to the sandbox. offprompt has no server of its own and keeps nothing.

What does it cost?

Nothing. offprompt is open source under the MIT licence, and npx offprompt init is the whole setup.

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